Simple Relaxation oscillator not working

OBW0549

Joined Mar 2, 2015
3,551
DISREGARD THE FOLLOWING. TOO EARLY IN THE MORNING, NOT ENOUGH COFFEE.

Expecting the drain of M2 to move from about 0v to 6v but not working
What constitutes "working"? What did you expect this circuit to do?

Your circuit will either sit there with the output of U1 at 1/2 of Vcc, or it will sit there with the output of U1 at 1/4 of Vcc, depending on the Vgs(thr) of M2 (which can range from 0.8 volts to 3.0 volts).

It won't do anything else.
 
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Ian0

Joined Aug 7, 2020
491
I'm guessing that your supply must be 6V?
I don't know why it shouldn't work. You should get a vaguely triangular wave at the junction of C1/R1 with a negative peak of 1.5V and a positive peak of 3V; and a 6V/0V squarewave on the output of the AD822 at about 3.8kHz.
What IS it doing? What are the DC voltages that it is stuck at?
Why did you include M1, M2 and R2 - you could have connected R5 directly to the AD822 output? That would make it the standard comparator relaxation oscillator (same principle as a 555).
Why are you using a £9 op-amp to do the same job as a 25p timer?
 

Ian0

Joined Aug 7, 2020
491
I see the Vcc voltage source in the bottom right corner now. It's 12V. The drain of M2 will got to Vcc when M2 is off.
If you want a 0-6V output, then make Vcc 6 Volts.
Why did you expect it to go to 6V?
 
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